Electrochromism is the ability of a material to change its electromagnetic radiation absorptive properties with the application of a current or voltage. The most practical color change is from an absorptive colored state to a colorless transmissive state, which allows shades to be turned on and turned off completely upon application of a current. Many materials, for example, transition metal oxides, Prussian Blues, some small organic molecules, and some π-conjugated polymers, demonstrate electrochromism. Electrochromic materials are useful for many technologies including displays, signage, windows, security, camouflage, and wearable fabrics.
The development of electrochromic windows or eyewear having panes or lenses that can switch from black or dark colored states to nearly colorless transmissive forms has been dominated by electrochromic metal oxides. A typical metal oxide based electrochromic device has a colored state resulting from a cathodically coloring tungsten oxide to absorb long wavelength light and an anodically coloring nickel oxide or vanadium oxide to absorb short and medium wavelength light in a color-mix that achieves an aesthetically pleasing and highly black color-neutral hue. These devices from transition metal oxides typically require 20 seconds or more to complete a full switch. While the switching speed is of little concern for architectural window-type electrochromic devices (ECDs), such speeds are not viable for applications such as automotive/aircraft windshields or pilot/athletic visors, where sub-second color to colorless switching is necessary or desired. Metal oxide electrochromic windows unfortunately possess a considerable degree of residual absorption in their transmissive states.
In contrast to metal oxides, a-conjugated electrochromic polymers (ECPs) have demonstrated a facility for color tuning due to polymer structural modification, color mixing via solution blends of polymers, and variations of device architectures. Because ECPs are typically cast as porous amorphous films, they exhibit an ability to switch rapidly with high contrast, with typical switching times ranging from two seconds to less than one second. ECPs have advantages for processing the devices, as thin films can be prepared using roll-to-roll techniques where solubility of the film is diminished by post-deposition functionalization. ECPs that are water soluble and display fast electrochromic switching are available.
To match the color neutrality achieved by metal oxides using a single ECP, complex backbone architecture has been developed where donor-acceptor (D-A) interactions permit absorption of long wavelength light and random length donor segments absorb medium to short wavelength light. ECPs with a high spectral broadness was first reported in Beaujuge et al., Nat Mater 2008, 7 (10), 795-9, where a random oxidative copolymerization of 3,4-propylenedioxythiophene (ProDOT) monomer with a ProDOT-benzothiadiazole (BTD)-ProDOT trimer was carried out. This “ECP-Black” displays long and medium runs of ProDOTs absorbed moderate wavelength light while the ProDOT-BTD-ProDOT segments absorbed short wavelengths. Greater color neutrality was then achieved by broadening of the absorption spectrum in the visible region, as reported in Shi et al. Advanced Materials 2010, 22 (44), 4949-53, where a Stille polymerization permitted shorter segments of ProDOTs in a chain that permitted more efficient capture of shorter wavelengths of light to achieve a more completely black neutral state. Subsequently, additional chemically polymerized black-to-transmissive ECPs have been achieved. Similar broadening effects for black-to-transmissive electrochromism have been achieved through the electropolymerization of monomers that utilize donor-acceptor (D-A) interactions, by making polymer blends, or by coupling poly(3,4-ethylenedioxthiophene) (PEDOT) with a yellow organic dye.
Like metal oxides, the black-to-transmissive polymers and devices display residual absorption in their transmissive states, which results in a less than desirable contrast across the visible spectrum during electrochromic switching. Many ophthalmic tints and coatings require an integrated contrast (Δ % Tint) across the visible of ˜50%, which is a value that ECP-Black is unable to achieve. Hence, the identification of ECPs with greater integrated contrast and their implementation in devices is desired.